I got some PTFE bearings from the dealer yesterday. It caused quite a stir. The parts department didn't think it was possible to buy two items from Mercedes that have a total cost under one euro including sales tax (VAT / BTW)! But after several checks they relented and let me pay my 82 euro cents and leave...
A visual inspection shows little difference in thickness between new and old. I can't measure them easily with the bunt nosed ends of my micrometer - and I don't want to damage the new ones (they cost a fortune!)...
...so I don't think the wear parts are the PTFE bearings in these differentials.
I have noticed, however, the metal thrust washers do have wear lips on their outer circumferences =>
I've kept the shims with the cogs as they came out of the casing and the measurements are 1.34mm for the one next to the crown wheel and 1.3mm for the other one